Work machines for moving mass quantities of earth, rock, minerals and other material typically comprise a work implement configured for loading, such as a bucket controllably actuated by at least one lift and one tilt hydraulic cylinder. An operator manipulates the work implement to perform a sequence of distinct functions. In a typical work cycle for loading a bucket, the operator first maneuvers close to a pile of material and levels the bucket near the ground surface, then directs the machine forward to engage the pile.
The operator subsequently raises the bucket through the pile, while at the same time "racking" (tilting back) the bucket in order to capture the material. When the bucket is filled or breaks free of the pile, the operator fully racks the bucket and lifts it to a dumping height, backing away from the pile to travel to a specified dump location. After dumping the load, the work machine is returned to the pile to begin another work cycle.
It is increasingly desirable to automate the work cycle to decrease operator fatigue, to more efficiently load the bucket, and where conditions are unsuitable for a human operator. Conventional automated loading cycles however, using predetermined position or velocity command signals, may be inefficient and fail to fully load the bucket due to the wide variation in material conditions. Pieces of interlocking broken rock left by blasting, referred to herein as "shot rock", and sedimentary earth, referred to herein as "hard pack", present particularly challenging material conditions. Power limitations of the machine hydraulic system may even make conventional automatic loading impossible when the bucket tip encounters larger rocks.
U.S. Pat. No. 3,782,572 to Gautler discloses a hydraulic control system which controls a lift cylinder to maintain wheel contact with the ground, by monitoring associated wheel torque. U.S. Pat. No. 5,528,843 to Rocke discloses a control system for capturing material which selectively supplies maximum lift and tilt signals in response to sensed hydraulic pressures. International Application No. WO 95/33896 to Daysys et al. discloses reversing the direction of fluid flow to the hydraulic cylinder to when bucket forces exceed allowable limits. None of the systems however, variably control the magnitude of the command signals in order to more efficiently capture material.
The present invention is directed to overcoming one or more of the problems as set forth above.